Comprised air brake booster with enhanced power-resist

ABSTRACT

In order to provide additional pneumatic boosting which comes into play only for heavy braking, provision is made, in a pneumatic brake-booster, for some boosting force ( 28, 33 ) which is in addition to a boost force ( 25 ) normally applied by a reaction disk ( 24 ) to be absorbed. In a preferred example, force is absorbed by means of a slide ( 29 ) containing an envelope ( 31 ) and a plunger ( 30 ). The plunger is rigidly fixed to the first reaction device. The envelope rests against a piston ( 23 ) of the pneumatic booster. The plunger is expelled from the envelope by a spring ( 34 ), the excursion of which is countered by a peg ( 35 ) as long as the force applied by a driver is below an intermediate force. Beyond that, the reaction of a hydraulic braking circuit downstream compresses the spring and causes the additional pneumatic boosting to come into effect.

[0001] The subject of the present invention is a pneumatic brake-boosterwith an enhanced boost effect. It is also an object of the invention toprovide greater boosting in the event of heavy braking. It relates topneumatic brake-boosters, the braking law of which has a point ofinflection when passing from normal braking, for which boosting isnormal, to heavy braking, for which boosting needs to be greater, with,furthermore, additional boosting.

[0002] A pneumatic brake-booster comprises, in principle, avariable-volume front chamber separated from a rear chamber, the volumeof which is also variable, by a partition formed by a sealed andflexible diaphragm and by a rigid skirt plate. The rigid skirt drives apneumatic piston bearing, via a push rod, on a primary piston of amaster cylinder of a hydraulic braking circuit, typically a tandemmaster cylinder. The front chamber, on the master cylinder side, isconnected pneumatically to a source of vacuum. The rear chamber, on theopposite side to the front chamber, and placed on the brake pedal side,is connected pneumatically in a way controlled by a valve, to a sourceof driving fluid, typically air at atmospheric pressure. At rest, thatis to say when a driver is not pressing on the brake pedal, the frontand rear chambers are connected together by a first valve, whereas therear chamber is isolated from atmospheric pressure by a second valve.Under braking, the front chamber is first of all isolated from the rearchamber by closing the first valve, then air is let into the rearchamber by opening the second valve. This letting-in of air has theeffect of driving the partition and of providing pneumatic boosting ofthe braking.

[0003] The force of a driver, transmitted by a brake pedal, and theforce of the brake-booster combine in the pneumatic brake-booster on areaction disk. On this disk, these two forces are applied to twoadjacent surfaces, generally concentric, in fixed proportions (thussetting a constant brake boosting ratio). The disk is also in reaction,particularly via a cup which contains it, with a reaction force exertedby a hydraulic braking circuit located downstream of the transmission ofthe forces.

[0004] The problems encountered under braking, particularly under heavybraking, are associated with driver behavior. What happens is that ifbraking has to be heavy, the brake boosting in effect needs to be great.For example, a proportion between a (significant) force applied by adriver and the force applied correspondingly by a pneumaticbrake-booster on a hydraulic brake operator has therefore to have a highvalue, for example higher than ten. By contrast, particularly in thecontext of vehicles driving through city traffic, such a high valueleads to a jolting ride which is unpleasant for the passengers and forthe driver. There are then two conceivable solutions.

[0005] Either the proportionality ratio adopted for braking is some waybetween a low ratio, which is useful for easily metering low brakingforces, and a high ratio, which is useful when braking is heavy. Such asolution is not satisfactory because it is not really suited to eitherof the two situations. Or, on the other hand, the brake boosting ismodified, particularly by adding additional hydraulic boosting, so as toprovide the braking, above and beyond a certain braking force, with aboosting proportionality which is higher than the proportion used whenthe force is low. In the latter case, a boost curve showing the forceapplied to a downstream hydraulic braking circuit as a function of theforce applied by a driver has a point of inflection above and beyond acertain value of this driver force: its slope becomes steeper. Thiscurve then experiences a known boost saturation effect which isencountered when the moving partition is brought as far as possible intothe front chamber and or when the rear chamber is raised to the highestpressure available (that of the ambient air). The second solution hasthe disadvantage of entailing the development of pneumaticbrake-boosters of new design, and of not being able to be fitted topneumatic brake-boosters which have already been produced. It is alsoknown that developing a new solution leads to a period of developmentand homologation, the duration and cost of which are not insignificant.Furthermore, the complexity of the new solution may in itself be afactor in high additional costs.

[0006] The invention seeks to solve these problems by creating a simplemechanical relay. Quite simply, above and beyond a certain brakingforce, provision is made for moving gear that transmits the brakingforce (at least one of its parts) to be deformed so that this gear comesto bear against the partition at a point other than via the reactiondisk. By proceeding in this way, at the time of this additional bearing,the proportionality ratio fixed by the ratio of the surface areas isbroken. In so doing, two functions can be fulfilled using one and thesame mechanism. A first function is that of detecting above and beyondwhat given value of the force applied by the driver the greater boostingneeds to be implemented. Another function is that of fixing, using thismechanism, another proportionality ratio which is better for heavybraking.

[0007] One principle of the invention may involve the use of thecompressible nature of the reaction disk. What happens is that underconventional boosted braking, the reaction disk becomes deformed underthe effect of the braking. The crushing of the various surfaces of thedisk leads to the opening and closing of one or other of the valves sothat the forces can reach equilibrium in the desired proportion.However, even though the disk acts like a fluid subjected to equalpressures on its surfaces (which fixes the proportionality ratio), itdoes not need to be made of a noncompressible material, and in actualfact it is not: it is made of a compressible material, for examplerubber.

[0008] As a result, when the disk is compressed, the moving gear isdeformed (broadly speaking contracts). In particular, a reaction cupwhich contains the reaction disk moves (in relative terms) in thedirection of the brake pedal. In the invention, it may be decided, for agiven compression value, to cause the brake-booster to absorb someforce. In other words, when a brake rod carried along by the brake pedalor the push rod are depressed by an amount 6 (corresponding to the forcefor which the boost ratio is to be increased), the moving partition isdepressed by an amount δ−ε, with ε small and of no consequence on thedynamics of the braking. This difference ε is then used to cause themoving partition to come into additional bearing contact against thepush rod of the moving gear.

[0009] The moving partition then acts on the moving gear in two ways andthese two actions combine their forces. On the one hand, in aconventional way, the reaction disk drives the moving gear along. On theother hand, the absorbed force supplements the thrust.

[0010] For this additional thrust to be filled, however, as aprogressive force by the driver, provision is also made for aproportional reaction to be integrated, this having the same effects asthose produced by the disk. What happens is that if such a proportionalreaction were not provided, then above and beyond a certain force on thepart of the driver, braking would be imposed only by the position of thefoot of this driver (without additional force). This could result inerratic braking.

[0011] A subject of the invention is therefore a pneumatic brake-boostercomprising:

[0012] a front chamber that can be connected to a source of vacuum,

[0013] a rear chamber that can be connected to a high-pressure inlet;

[0014] a sealed moving partition between the two chambers,

[0015] a brake rod,

[0016] a hydraulic braking circuit,

[0017] a moving gear, this moving gear being carried along, on the onehand, via a first reaction device, via the moving partition and via thebrake rod, and, on the other hand, being in reaction, via the firstreaction device, with the hydraulic braking circuit, and

[0018] a device for letting a high-pressure fluid into the rear chamberat the time of braking, characterized in that it comprises:

[0019] a second reaction device for carrying the moving gear along viathe moving partition.

[0020] The invention will be better understood upon reading thedescription which follows and from examining the accompanying figures.These are given only by way of non-limiting indication of the invention.The figures show:

[0021]FIG. 1: overall view in section of a pneumatic brake-boosteraccording to the invention;

[0022]FIGS. 2a and 2 b: curves representing the braking forces withconventional pneumatic boosting and in the invention, respectively;

[0023]FIGS. 3a and 3 b: sections of a preferred alternative form ofembodiment of the pneumatic brake-booster of the invention, in twomutually perpendicular orientations.

[0024]FIG. 1 shows a pneumatic brake-booster with enhanced boost effectaccording to the invention. This pneumatic brake-booster comprises afront chamber 1 that can be connected by a plug 2 to a source of vacuum,not depicted. Typically, the source of vacuum may consist of an inletgas tapping for a vehicle with a petrol engine. In the case of a vehiclewith a diesel engine, use would be made of an external source of vacuum.The pneumatic brake-booster also comprises a rear chamber 3 that can beconnected, for example by a valve 4, to a high-pressure inlet 5(typically air at ambient pressure). The pneumatic brake-booster alsocomprises a moving partition 6 habitually equipped with a rigid skirtand with a sealed diaphragm. The diaphragm prevents pneumaticcommunication between the two chambers. The partition 6 is pierced witha sealed orifice 7 to allow a moving gear 8 to pass. The moving gear 8is connected mechanically, on the one hand, by a brake rod 9 to a brakepedal and, on the other hand, to a hydraulic braking circuit 10. Theprinciple of the boosting afforded by such a pneumatic brake-booster isas follows. Under the action of the rod 9, the moving gear 8 plungesinto the rear chamber, uncovering the valve 4 via which ambient air islet into the rear chamber 3. The ambient air then exerts pressure on thepartition 6 which, in addition to the action of a thrust face 11 securedto the rod 9, drives the moving gear 8 in such a way that one end 12thereof actuates the hydraulic braking circuit 10.

[0025] Such a system is known and leads to a braking law, curve 13 inFIG. 2a, in which a force F1 applied by a driver to the rod 9 is echoedin a force F2 applied by the moving gear to the hydraulic brakingcircuit 10. The curve 13 shown indicates that above and beyond athreshold force Fs, the brake boosting effect follows a correspondencelaw 14. It also shows that above and beyond a force FS, known as thesaturation force, the brake boosting effect produced by the pneumaticbrake-booster then merely follows a correspondence law 15 because themoving partition 6 can no longer move in the direction of the frontchamber 1, either because it has reached the limit of this chamber orrather because the rear chamber 3 is completely subjected to atmosphericpressure. In a known way, the saturation force has to be situated aboveand beyond a force Fb applied to the hydraulic circuit for which thevehicle wheels lock up. The value of the force Fs is set by choicescorresponding to driver comfort, in particular allowing him to rest hisfoot on the brake pedal without braking. The boost slope 14 for aconventional pneumatic brake-booster is linear and depends essentiallyon respective surface areas over which the brake rod and the movingpartition bear on a reaction disk.

[0026] By comparison, FIG. 2b shows the improvement afforded by theinvention. The curve giving the correspondence between the force appliedby the driver and the force applied to the hydraulic circuit experiencesa first evolution 16 comparable with the evolution 14 of curve 13.However, above and beyond a given force value Fi which is intermediatebetween the threshold force Fs and the saturation force FS, themechanism of the invention provides additional boosting so that theboost slope comprises a much more angled segment 17. As a result, thesaturation force FS′ encountered with the device of the invention is farlower than the saturation force FS of the conventional solution. Aboveand beyond the saturation force FS′, the correspondence curve has asegment 18 similar to the segment 15. It will be seen later on how it ispossible to adjust the value of the intermediate force Fi.

[0027] This adjustment leads to the creation of a segment 16 and asegment 17 or, in some cases, for example for sports cars, to theoffering, from the outset, of boosting, the slope 19 of which is thesame as that of the segment 17. Where segments 16 and 17 meet there is apoint of inflection 20 at which the boost force increasesproportionately far more swiftly.

[0028] The additional balanced reaction mechanism preferably added bythe invention is intended to prevent the slope 17 from becoming infinite(in practice far too steep) along a vertical line 21. To give a concreteexample, the proportionality ratio for the segments 14 and 16 may beallowed to be of the order of four to six, whereas in the case of thesegments 17 or 19 it could reach a ratio higher than ten.

[0029]FIG. 3a shows a detail of a first known reaction device and,according to the invention, a second reaction device. The moving gear 8is carried along by the moving partition 6 by the effect 22 of air atatmospheric pressure let in through the valve 4. The partition 6 bears,via a hollow pneumatic piston 23, against a flexible disk 24. The disk24 is contained in a cup 25 perfectly tailored to its shape. The piston23 has the overall shape of a bell. The partition 6 bears against theexterior base of the bell, and the top of the piston 23 is pierced andhas an interior peripheral ring 26 which bears on the disk 24 at theinterior periphery of the cup 25. One end 27 of the brake rod 9 pressesexactly inside the ring 26. The end 27 forms the thrust face 11. Thering 26 and the end 27 are such that between them they occupy all of thespace inside the cup 25.

[0030] The disk 24, acting like a fluid, allows the (very minimal)relative displacement of the end 27 with respect to the base of the ring26 to be organized. Under braking, the end 27 first of all penetratesthe disk 24, which is flexible. In doing so, the valve 4 at the rear ofthe pneumatic brake-booster opens, air enters the rear chamber 3. Viathe partition 6, the piston 23 presses in its turn against the disk 24.This phenomenon occurs until such time as the bearing forces once againreach equilibrium. When this equilibrium is reached, the disk is flat.As long as it is not reached, the valve 4 remains open and boostingoccurs. In practice, for each moment of the braking, along the segment14 or along the segment 16, the forces applied by the end 27 and by thering 26 reach equilibrium because of the plasticity of the disk 24. Theresult then obtained is that, on the one hand, at the time of braking,this end 27 and this ring 26 are almost always at the same place onewith respect to the other and that, overall, the cup 25 is pushed in thedirection of the hydraulic circuit 10. The slope of the segments 14 and16 is linked by the proportion of the surface areas of the crosssections of the ends 27 and of the base of the ring 26, respectively. Asthis proportion does not vary at all throughout braking, it will beunderstood that boosting is linear.

[0031] In the invention, in addition to the first reaction deviceconsisting of the disk 24, a second reaction device is created whichwill allow another equilibrium between additional (far greater) boostingand the forces applied to the brake rod (in this case also boosted bythe first reaction device). To achieve this, the piston 23 has, insidethe bell, a circular thrust face 28 forming a ledge. The moving gear 8therefore comprises, in the preferred exemplary embodiment, a slide 29formed of a plunger 30 and of an envelope 31. The plunger 30 is solidlyfixed to the base of the cup 25. It extends from this cup in thedirection of the hydraulic circuit 10. The envelope 31 envelopes theplunger, drives the moving gear in the downstream direction, and at itsbase has a saucer 32, the peripheral ends 33 of which can bear againstthe ledge 28. At rest, there is a small clearance between the ends 33and the ledge 28. Also placed, in compression, between the envelope 31and the cup 25 is a helical spring 34. The spring 34 surrounds theplunger 30. Because of the action of this spring 34, the plunger 30 isnormally expelled from the envelope 31.

[0032] To simplify the assembly of the parts, provision is thereforemade for the plunger 30, FIG. 3b, to be fitted with a peg 35 whichprotrudes into a slot 36 made in the envelope 31. As the spring 34 is incompression, the peg 35 bears against the base of the slot 36.

[0033] The helical spring 34 is compressed between the saucer 32 and thecup 25 with a force Fc (FIG. 2b) corresponding to the intermediate forceFi for which the intervention of the additional pneumatic boosting is tobe set. In other words, for any action applied by a driver, the force ofwhich is below the intermediate force Fi, the assembly consisting of theplunger 30, the envelope 31 and the spring 34, behaves rigidly. Theforce from the driver and the boost force are therefore transmitted tothe hydraulic circuit along the segment 16.

[0034] As soon as the force applied by the driver becomes higher thanthe force Fi, the spring 34 is no longer held by the peg 35 in the slot36 but on the other hand is compressed by the braking force on the onehand and by the reaction of the hydraulic circuit, on the other hand.Being compressed beyond the force Fc, the spring 34 shortens and theperiphery 33 of the saucer 32 comes to bear against the ledge 28. In sodoing, the boosting contribution is afforded, on the one hand, by thering 26 (as before) but also by the saucer 32 through the bearing of theledge 28.

[0035] It may be, once a small clearance, for example, of one millimeteror half a millimeter present between the periphery 33 and the ledge 28(corresponding to the clearance of the peg 35 in the slot 36) has beentaken up, that any additional action applied by the driver on the brakepedal and therefore on the end 27 leads to a corresponding movement ofthe saucer 32, of the envelope 31 and therefore accordingly of themoving gear as a whole. In this case, the additional braking would beafforded with a zero (or practically zero) marginal force. If thisapproach were taken, the additional boosting would then have the slopeof the half straight line 21 in FIG. 2b. That could be too steep. Itwill be noted that if there is nonetheless the desire to adopt such asolution, all that would be needed would be for the mechanism to beassembled without the spring 34, without the plunger and without theenvelope, and with simply a saucer 32 fixed to the moving gear and forthe compression of the disk 24 to be used to lead to the saucer 32 beingmoved closer to the ledge 28.

[0036] In the invention, it has been preferable to provide additionalpneumatic boosting of which the slope, while being far steeper, is notexaggerated. To this end, the saucer 32 is slightly flexible andbehaves, inserted between the envelope 31 and the ledge 28, like aspring. The slope of the segment 17 is therefore equal to the ratiobetween the stiffness of the spring equivalent to the saucer 32 and thestiffness of the spring 34 (or of the spring 34 and of a springequivalent to the compressibility of the disk 34).

[0037] To give a concrete example, the force Fi applied by a driver isof the order of 100 kilos. The boosting afforded by the ring 26 alone isof the order of 3000 DaN, leading to a force applied at the mastercylinder Fc of the order of 4000 DaN. It may therefore be allowed thatfor a force only very slightly higher applied by the driver, for example1100 DaN (100 DaN more), the force transmitted by the cup 25 is 4400 DaN(3300 DaN plus 1100 DaN) and that the force transmitted by the saucer 32is 1200 DaN. In this case, the slope of the segment 17 would correspondto a force ratio of 15. In this example, the compression of the spring34 would be of the order of 4000 DaN. Furthermore, the stiffer thesaucer 32, the steeper the slope 17.

[0038] With the height of the slot 36, the diameter and force of thespring 34, the stiffness of the saucer 32 and the compressibility of thedisk 24, there are various ways of adjusting both the value of theintermediate force Fi above and beyond which additional boosting willcome into play and the slope 17 of this additional pneumatic boosting.Of course, it will be noted (FIG. 1) that the bell-shaped piston 23 ofthe state of the art already had a ledge 28 which in practice was usedto hold a spring 37 for returning the partition 6 to move the latteraway from the front chamber at the time of brake release. Furthermore,saucers such as 32 were already provided in the state of the art.However, these were used only to keep the various parts of the movinggear more or less aligned. By comparison with the known solutions in thestate of the art, the invention now introduces the idea of theclearances between the saucer 32 and the ledge 28 and the thrust faces11 of the end 27 of the rod 9 on the moving gear no longer being leftindependent of one another but designed so that forces can be absorbed.In practice, with the preferred solution adopted, the invention mayconsist in replacing an upstream end of the moving gear 8 with the slide29 bearing via the spring 34 on the back of the cup 25. Suchmodifications are entirely compliant with the known embodiments and donot lead to tricky development or homologation work.

[0039] It will be noted that the spring 34 and the spring formed by thesaucer 32 are inserted in series between the moving partition 6 and thefirst reaction device consisting of the cup 25. The second reactiondevice formed by the envelope 31 is connected, on the one hand, to themidpoint of the two springs in series and, on the other hand, exerts itsforce on the hydraulic braking circuit.

1. A pneumatic brake-booster comprising: a front chamber (1) that can beconnected to a source of vacuum (2), a rear chamber (3) that can beconnected to a high-pressure inlet (5); a sealed moving partition (6)between the two chambers, a brake rod (9), a hydraulic braking circuit(10), a moving gear (8), this moving gear being carried along, on theone hand, via a first reaction device (24, 25), via the moving partitionand via the brake rod, and, on the other hand, being in reaction, viathe first reaction device, with the hydraulic braking circuit (10), anda device (4) for letting a high-pressure fluid into the rear chamber atthe time of braking, characterized in that it comprises: a secondreaction device (29-34) for carrying the moving gear along via themoving partition.
 2. The booster according to claim 1, characterized inthat the second reaction device comprises a first spring (34) and asecond spring (32), which springs are inserted in series between themoving partition and the first reaction device, the second reactiondevice being mechanically connected to the midpoint of the two springsin series.
 3. The booster according to claim 2, characterized in thatthe first spring is helicoidal, the second spring being formed by asaucer.
 4. The booster according to claim 3, characterized in that, atrest, the saucer is separated from the partition by a clearance.
 5. Thebooster according to claim 2, characterized in that the moving gear isconnected to the reaction devices via a slide (29).
 6. The boosteraccording to claim 5, characterized in that the slide comprises aplunger (30) connected rigidly to the first reaction device and anenvelope (31) connected to the midpoint of the springs and to the movinggear.
 7. The booster according to claim 6, characterized in that theplunger comprises a protruding peg (35) and in that the envelope has aslot (36) to allow the peg to pass and to keep the moving plunger inposition with respect to the casing.
 8. The booster according to claim2, characterized in that the springs have stiffnesses which are set sothat intervention of the second reaction device occurs for a force (Fi)the value of which is intermediate between a threshold force (Fs) fortriggering boosting by the first device and a saturation force (FS).